In recent years, the use of secondary batteries such as lithium-ion batteries are rapidly increasing in, for example, power storage devices and storage batteries for vehicles in which new energy systems such as solar cells and wind power generation are combined. When many power storage elements such as unit batteries (also called battery cells, single cells or cells) are used to generate high power, a configuration in which a plurality of power storage modules are connected in series is employed. A battery block is constituted by connecting a plurality of battery cells, for example, four battery cells, in parallel or in series. Many battery blocks are housed in a sheath case to constitute a power storage module (simply referred to as a module in the following description).
Furthermore, there is known a power storage system including a plurality of modules connected with each other and a common control device disposed to the plurality of modules. Each of the modules has a module controller, and the module controller communicates with the control device via a communication means.
Furthermore, when the power storage system is used as a power source, the module may be sometimes necessary to be switched to a backup module even while current flows. For example, since a power source cannot be switched off in an emergency power source, a power source at a mobile phone base station, and the like, the module is replaced while current flows. Such a mechanism of the device having a structure that allows attachment and removal while a power source remains on is called hot line insertion and removal (a hot swap).
In the hot swap, two modules MO1 and MO2 are connected in parallel via a switch S, as schematically illustrated in FIG. 1. The module MO1 includes a battery unit BAT1 and an internal resistance R1, while the module MO2 includes a battery unit BAT2 and an internal resistance R2. A storage battery module mounted with lithium-ion batteries had a problem that the module cannot be replaced during operation, in contrast to with lead storage batteries.
With respect to switching a power source to a backup power source, there have been proposed technologies disclosed in Patent Literature 1 and Patent Literature 2. In the technology disclosed in Patent Literature 1, outputs from two batteries are connected with a common power source circuit via field effect transistors. When one battery is carelessly removed while power is supplied from the battery to a load, another battery is switched to a state of supplying a power source to the load.
In the technology disclosed in Patent Literature 2, when a load device operating with direct current has a trouble of a power source, direct current backup power is supplied. Battery packs each including a plurality of secondary batteries connected in series are provided so that a voltage required by the load device can be obtained. Each of the battery packs is determined for life limit, and only the battery pack determined as having reached life limit is replaced.